1. Field of the Invention
The present invention relates to an optical system adapted for a photographic lens of a silver-halide photographic camera, a video camera, an electronic still camera, a digital camera or the like, and more particularly to a photographic optical system having a high optical performance and optimum for an automatic focusing operation in which focusing is performed by moving a small-sized and light lens unit disposed inside the optical system.
2. Description of Related Art
In general, a focusing operation in many photographic lenses is performed by moving, along an optical axis, the entirety of an optical system or a lens unit which is a part of the optical system.
Among the photographic lenses, in particular, a telephoto lens of long focal length has a tendency to make the size of the whole optical system large and the weight of the lens heavy as compared with a lens system of short focal length.
Accordingly, if, in the telephoto lens, focusing is performed by moving the entirety of an optical system, it becomes necessary to move a large and heavy lens a longer distance according to the long focal length, so that it becomes mechanically difficult to perform focusing speedily.
Therefore, in many telephoto lenses, focusing is performed by moving a lens unit which is a part of an optical system. In particular, there have been proposed a variety of telephoto lenses which adopt the so-called inner focusing method in which focusing is performed by moving a lens unit which is other than a front lens unit of the optical system, is relatively small in lens diameter and relatively light in lens weight, and is located in a middle portion or a relatively rear portion of the optical system.
For example, in Japanese Laid-Open Patent Application No. Sho 55-147606 (corresponding to U.S. Pat. No. 4,348,084), there is disclosed a telephoto lens of the inner focusing type having a focal length of 300 mm and an F-number of 2.8. In Japanese Laid-Open Patent Application No. Sho 59-65820 (corresponding to U.S. Pat. No. 4,610,514) and Japanese Laid-Open Patent Application No. Sho 59-65821 (corresponding to U.S. Pat. No. 4,610,514), there are disclosed telephoto lenses of the inner focusing type having a focal length of 135 mm and an F-number of 2.8 or thereabout.
Each of the telephoto lenses of the inner focusing type disclosed in the above references comprises, in order from the object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power and a third lens unit of positive refractive power, in which focusing is performed by moving the second lens unit along the optical axis.
In general, in telephoto lenses, as the focal length is made longer, or as the total lens length is made shorter for compact size, longitudinal chromatic aberration and lateral chromatic aberration are apt to increase to make the image forming performance worse. Therefore, there have heretofore been proposed a variety of telephoto lenses in which a front lens unit, having a paraxial on-axial ray and a pupil paraxial ray pass at relatively high positions from an optical axis, is composed of a positive lens of low dispersion and a negative lens of high dispersion having extraordinary partial dispersion and made of fluorite or the like.
Further, in many telephoto lenses designed to be mounted on automatic focusing cameras which have been actively developed in recent years, the so-called inner focusing method is adopted in which focusing is performed by moving, along an optical axis, a lens unit which is other than a front lens unit of the optical system, is relatively light in lens weight to make the load torque on a lens barrel small, and is located in a middle portion or a relatively rear portion of the optical system. In addition, in order to realize a higher-speed automatic focusing operation, it is strongly desired to further reduce the weight of a focusing lens unit.
In general, the inner focusing method has such advantages that, since a lens unit used for focusing is small in size and light in weight, the operability of the focusing lens unit is good and the high-speed operation thereof is possible, and there occurs little change of the center of gravity of the entire lens system between focusing on an infinitely distant object and focusing on a closest distance, thereby enabling the telephoto lens to be easily held.
On the other hand, if the inner focusing method is adopted in telephoto lenses, the variation of spherical aberration and chromatic aberration occurring during focusing is apt to increase. In particular, it is relatively difficult to correct well the variation of chromatic aberration, which is a cause of deteriorating the image forming performance when a close object is photographed.
Therefore, in many telephoto lenses of the inner focusing type, the focusing lens unit is composed of at least one positive lens and at least one negative lens so as to correct residual chromatic aberration caused by the front lens unit and the variation of chromatic aberration caused by the focusing lens unit itself during focusing.
However, the conventional focusing lens unit is too heavy in weight to perform a higher-speed automatic focusing operation.
It is a main object of the invention to provide an optical system having a focusing lens unit which is light in weight, and it is another object of the invention to provide an optical system capable of correcting well chromatic aberration and other aberrations and the variation of the various aberrations occurring during focusing, and, in particular, being optimum for an automatic focusing camera and having a high optical performance.
To attain the above objects, in accordance with an aspect of the invention, there is provided an optical system comprising, in order from an object side to an image side, a first lens unit of positive optical power, and a second lens unit of negative refractive power, wherein focusing from an infinitely distant object to a closest object is effected by moving the second lens unit toward the image side along an optical axis, and the second lens unit consists of one negative lens.
In the optical system in accordance with the aspect of the invention, it is desirable that the following conditions are satisfied:
d2L1(xcex)/dxcex2 greater than 0xe2x80x83xe2x80x83(1) 
d2T1(xcex)/dxcex2 less than 0xe2x80x83xe2x80x83(2) 
xcexd2 greater than 30xe2x80x83xe2x80x83(3) 
where
L1(xcex) is an aberration coefficient of longitudinal chromatic aberration of the first lens unit,
T1(xcex) is an aberration coefficient of lateral chromatic aberration of the first lens unit,
xcex is a wavelength of light, and
xcexd2 is an Abbe number of material of the negative lens of the second lens unit, being a xcexdd value expressed by xe2x80x9cxcexd2=(Ndxe2x88x921)/(NFxe2x88x92NC)xe2x80x9d where Nd, NF and NC are refractive indices with respect to d line, F line and C line, respectively.
The above and further objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.